cb: (x: T) => void not assignable to cb: (x: T) => void when it has an overload: "'any' is not assignable to 'never'"

TypeScript Version: 2.9.0-dev.20180411

Code

declare function provide<T>(cb: (x: T) => void): void;
declare function provide<T>(cb: (x: T[keyof T]) => void): void;

declare function provider<T>(provide: (cb: (x: T) => void) => void): void;
provider(provide);

Expected behavior:

No error.

Actual behavior:

src/a.ts(4,10): error TS2345: Argument of type '{ <T>(cb: (x: T) => void): void; <T>(cb: (x: T[keyof T]) => void): void; }' is not assignable to parameter of type '(cb: (x: never) => void) => void'.
  Types of parameters 'cb' and 'cb' are incompatible.
    Types of parameters 'x' and 'x' are incompatible.
      Type 'any' is not assignable to type 'never'.

Worked in typescript@2.9.0-dev.20180327, broken in typescript@2.9.0-dev.20180328.

I've opened a PR with a potential fix for the first, and opened a new issue for the second (since it seems to have a separate root cause) so this issue can just track the first.

The fix here seems to have nonzero perf impact and we haven't seen any other reports. Punting for now.

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`cb: (x: T) => void` not assignable to `cb: (x: T) => void` when it has an overload: "'any' is not assignable to 'never'" #23352

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cb: (x: T) => void not assignable to cb: (x: T) => void when it has an overload: "'any' is not assignable to 'never'" #23352

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weswigham commented on Oct 12, 2020

RyanCavanaugh commented on Feb 11, 2022

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`cb: (x: T) => void` not assignable to `cb: (x: T) => void` when it has an overload: "'any' is not assignable to 'never'" #23352